Digital receivers are known. Proponents of digital receivers typically cite their immunity from operational variations due to temperature, humidity, and component aging as the primary advantages over their analog receiver counterparts. The primary technological advance that has spurred development of digital receivers is the Digital Signal Processor (DSP). DSPs are readily programmable to perform many combined functions and features. For example, a digital intermediate frequency (IF) filter may be made programmable in terms of channel frequency, sampling rate, and the desired filter response. Additionally, a DSP, executing alternately stored programs, may perform many different types of filtering and demodulation to implement completely different types of receivers. Further, DSPs facilitate miniaturization since, in one (or a few) Large Scale Integrated (LSI) packages, many analogous analog functions may be performed.
To obtain the maximum benefit from the use of digital technology, most digital receiver designers strive to digitize a received signal as soon as possible, preferably, before the first mixer and IF. However, the bandwidth in these early stages of a typical receiver require extremely high sampling rates to properly digitize the received signal in accordance with the Nyquist theorem. For example, a receiver having a 30 MHz "front end", would require a sampling rate in excess of 60 MHz. The circuits and components used to digitize a received signal at extreme rates consume tremendous amounts of power so as to render a portable receiver implementation, and to a certain extent a mobile receiver implementation, impossible or impractical due to the unacceptably high current drain on the battery. As used herein, a mobile receiver is a receiver designed to be installed in a vehicle, and a portable receiver is a receiver designed to be carried on or about the person.
Accordingly, some digital receiver designers opt to digitize the received signal in the "back end" of the receiver, which is generally understood to be after the IF selectivity. Since the bandwidth and dynamic range requirements of these later stages are reduced, the sampling rate may be lowered to conserve power consumption. Generally, this eases the burden of providing a mobile receiver, however, advantageous DSP and receiver design techniques such as varying the IF bandwidth and filter shape are prohibited. Accordingly, a need exists to provide a digital receiver that operates in the mobile and portable environments.